The prevalence of foreign trace DNA
A review of the available data
Felix Bittner
A literature thesis presented as part of the degree of
Master of Science
Supervisor: Bas Koksoorn, PhD
Co-assessor: Prof. Ate Kloosterman, PhD
Institute for Interdisciplinary Studies
University of Amsterdam
Netherlands
Date: 15-08-2018
The prevalence of foreign trace DNA
A review of the available data
Felix Bittner
Abstract
Data on the prevalence of foreign trace DNA is important when
forensic scientists evaluate the probability of evidence given
hypotheses such as indirect transfer or background DNA presence.
This work evaluated the literature produced since the last review on
this topic. Even though a number of studies are now available,
problems remain. Data is highly specific to experimental
circumstances and not widely applicable. On top of that, data are
seldom reproduced and no consensus on how to report data exists.
Nonetheless the literature indicates that many items hold foreign
trace DNA and, consequently, DNA samples created by touch often
show foreign alleles. Peak height differences between donors and
foreign trace DNA seem to exist often and may be a future basis of
interpretation. We recommend that currently available literature be
only used as as an indicator of evidential strength and limitations
should be reported accordingly. Future work should focus on large
scale, collaborative studies trying to capture data on foreign trace
DNA from the general population rather than in highly specific
scenarios. Alongside, reporting standards and collaborative
databases should be implemented.
Acknowledgements
I want to thank all those who have lend their guidance in the
writing of this thesis. In particular I want to thank Bas
Kokshoorn and Ate Kloosterman for their advice and
enormous patience.
Contents
1 Introduction 1
2 Search methodology 4
3 Results & Discussion 5
3.1 The prevalence of foreign trace DNA . . . 5
3.1.1 Foreign trace DNA on hands . . . 5
3.1.2 Foreign trace DNA on clothing . . . 5
3.1.3 Foreign trace DNA on public objects . . . 7
3.1.4 Foreign trace DNA in police laboratories . . . 8
3.2 The origin of foreign trace DNA . . . 8
3.2.1 Foreign trace DNA through laundering . . . 8
3.2.2 Foreign trace DNA through evidence handling . . . 10
3.3 The composition of trace DNA left behind after contact . . . 10
3.3.1 Transfer through touch . . . 10
3.3.2 Transfer through touch after handshake . . . 11
3.3.3 Transfer through touch after non-intimate social contact . . . 13
3.3.4 Transfer by touch after touching previously used objects . . . 13
3.3.5 Transfer through touch after contact with used textiles . . . . 14
3.3.6 One time use of regularly used objects . . . 14
3.3.7 Complex scenarios . . . 15
3.4 Summarising discussion . . . 16
List of Figures
3.1 Average number of contributors to DNA profiles on public objects. . . 7 3.2 Percentage of samples with at least one foreign allele on hands and
deposited by touch. . . 11 3.3 Composition of DNA profiles recovered from cable ties by 4 different
List of Tables
2.1 Number of results and included number of articles per search term. . 4 3.1 Numbers of foreign trace DNA studies published since 2013 . . . 17 3.2 Comparison of foreign DNA prevalence after contact of interest. . . . 18
Chapter 1
Introduction
’Who is involved?’ is one of the central questions in criminal investigation in both fiction and professional manuals such as Fisher and Fisher (2012). In 1985, Sir Alec Jeffreys created a powerful tool that helps to answer this question - DNA ’fingerprinting’ Jeffreys, Wilson and Thein (1985). By comparing markers in the DNA of a crime-scene- to a reference-sample, scientists could now infer the donor of the scene-sample with a statistical likelihood far exceeding blood group typing and other methods. Unsurprisingly, the prevalence of biological tissue in crime has sparked an enormous interest in forensic DNA typing by scientists and police forces alike.
From 1985 to present, the sensitivity of forensic DNA typing has multiplied man-ifold so that investigators can now, under ideal conditions, produce a DNA profile from a single diploid cells ( 6pg of DNA) (Kloosterman & Kersbergen, 2003).This sensitivity has advantages, however also leads to challenges. In a forensic context, associating a person with a DNA sample is less interesting than associating a per-son with, for example, a drop of blood or semen. Even with large samples this association of DNA to a cell type is complicated but reasonable inferences can be made (de Zoete, Oosterman, Kokshoorn & Sjerps, 2016). With very small samples this becomes more and more complicated, if not impossible, and consequently their relevance can be contested in a court setting. These low quantity samples without known cell type are commonly categorised as ’trace DNA’.
Low amounts of DNA are often transferred by touch and much of the trace DNA literature has focused on this. However, as G. Meakin and Jamieson (2013) point out, trace DNA may, for example, also be deposited by speaking, coughing or sneez-ing. Generally, DNA from touch/contact will often be trace DNA, but trace DNA will not necessarily be a result of touch/contact. Nonetheless, ’touch DNA’, or other synonyms such as ’contact traces’ are often mixed up with the term trace DNA. Throughout this text the following definitions will be used to prevent any confusion:
Trace DNA - DNA without a known tissue of origin.
Touch DNA - DNA deposited by touch, or the interaction of two surfaces. Note that in casework the term touch DNA may be especially undesirable due to its suggestive nature.
Despite the challenges, forensic laboratories now routinely analyse trace DNA samples. At the same time, forensic DNA analysis has moved beyond the source and towards the activity that has lead to a DNA deposition. In a, so called, ’activity level analysis’ the scientist weighs up the probabilities of observing evidence, in this
The prevalence of foreign trace DNA - A review of the available data
case a DNA profile with a known quantity and quality, given competing hypotheses about activities. It may for example be more likely to observe a person’s blood (in large quantities) at the scene of crime given he was injured there (first hypothesis) rather than him never having been there (second hypothesis).
Commonly, four factors are help interpret DNA transfer given various activities: transfer, prevalence, persistence and recovery of DNA.Considering, for example, the hypothesis that a handshake took place between two individuals:
• How much DNA is transferred during a handshake of this type?
• How much DNA is present on somebody’s hands generally and from whom is it?
• How long does foreign DNA persist on somebody’s hand?
• What fraction of the DNA present on hands can be recovered with a certain methodology?
These data can then aid the scientist in estimating the probability of observing a certain amount of DNA from somebody on someone else’s hands given the hypothesis of a handshake.
With trace DNA, such an interpretation becomes extremely difficult. This is partly because the cell type is unknown but also due to the many possibilities of transferring small amounts of DNA onto an object. In recent years forensic scient-ists published many studies into transfer, prevalence, persistence and recovery of trace DNA and G. Meakin and Jamieson (2013) provide an excellent review. They concluded that neither quantity nor quality of the DNA present on an item would allow for a reliable inference of the mode of transfer based on the available data. Champod (2013) and Casey et al. (2016) represent a summary of the discussion on the merits of activity analysis with trace DNA that followed.
Champod (2013), points out that it is the duty of scientists to address activity level questions, as leaving it up to the trier of fact may lead to false conclusions. Any interpretation however would have to be based in relevant data, rather than the scientists experience, and may have to be focused on outlining limitations of said data. Casey et al. (2016), on behalf of the Body Fluid Forum, refer to published (and unpublished) data in combination with the experience of laboratories within the Body Fluid Forum, to support the interpretation of certain scenarios involving trace DNA. Contrary to Champod (2013) however, they also see a scientist’s general case experience of value to such an interpretation. Despite their disagreements on the value of experience, both mirror the general consensus within the forensic community that activity level analysis of trace DNA is the duty of a forensic scientist. The research into factors affecting trace DNA transfer, prevalence, persistence and recovery thus continues to be of major concern to forensic DNA analysis.
The prevalence of foreign trace DNA - A review of the available data
While G. Meakin and Jamieson (2013) focused their review mostly on the direct transfer of trace DNA, meaning transfer from the donor of the DNA directly to an object, they briefly touch on the issue of indirect transfer. This shifts the frame of reference from ’Who’s DNA is it and through which activity did they place it onto the item?’ towards ’Could somebody’s DNA be transferred to an item via another person(s) or object(s)?’. G. Meakin and Jamieson (2013), relying on the literature at that time, concluded that it would not be possible to weigh up direct vs. indirect transfer based on the amount or quality of DNA present on an item. However, their literature review reaffirmed the possibility of indirect DNA transfer that was already described by van Oorschot and Jones (1997).
Indirect transfer, together with the possibility of unrelated trace DNA being present before the events in question (background), pose a problem to the forensic scientist. In case work the true origin of observed DNA is unknown and multiple hypotheses have to be addressed by a statistical evaluation and as Champod (2013) pointed out, such evaluations are best made with experimental data.
Since 2013 a substantial amount of literature, partially or fully, addressing indirect transfer and prevalence of trace DNA has become available. Much of this information however is scattered over multiple articles and often is a secondary issue in the study in question. To make this knowledge more applicable to case work and to focus research this work will systematically review and summarise the literature on trace DNA (2013 - present) considering the following issue:
1. The prevalence of foreign trace DNA. 2. The origin of foreign trace DNA.
3. The composition of trace DNA left behind after contact.
This will provide forensic scientist with both a guide for future research as well as with a reference point for casework interpretation. And additionally will assess the progress that has been made since (G. Meakin & Jamieson, 2013).
Chapter 2
Search methodology
Articles were downloaded from the PubMed database by the US National Library of Medicine, National Institutes of Health. It contains over 28 million references from the biomedical field and spans books and life science journals.
Search procedure
To retrieve all literature available on trace DNA, the following stepwise approach was used. In the first step, PubMed was queried for the term ”trace DNA”, in the second step for the term ”touch DNA”, followed by ”DNA transfer” forensic. Search results were filtered for articles published after 15th of February 2013, to exclude articles addressed by G. Meakin and Jamieson (2013).
At each step articles were excluded if:
1. the article was already downloaded in a prior step.
2. the article did not include any reference to the topic of trace DNA in its title or abstract.
3. the article’s content laid outside the field of human DNA analysis.
In the next step, articles were scanned for content relating to the presence of foreign trace DNA by looking for keywords such as ’non-self DNA’, ’background DNA’, ’DNA mixtures’, ’unknown contributor’ etc. Articles with no reference to these, or related keywords, were excluded from the review.
Search results
The initial search results can be found in table 2.1. After scanning carefully for the above mentioned keywords, 29 articles were deemed relevant to this review.
Table 2.1: Number of results and included number of articles per search term.
Search term
Result number
Included articles
”trace DNA”
69
53
”touch DNA”
53
32
”DNA transfer” forensic
40
19
Chapter 3
Results & Discussion
3.1
The prevalence of foreign trace DNA
3.1.1
Foreign trace DNA on hands
Trace DNA transfer by direct touch is one of the frequent hypotheses in an activity level interpretation. An alternative hypothesis to direct transfer is often that the DNA was indirectly transferred via the hands of another person. For an evaluation of these hypotheses a clear understanding of the background level of foreign DNA on hands is necessary, as the scientist has to base his probability assignments on data.
To understand the composition of DNA on people’s hands better, Lacerenza et al. (2016) co-extracted DNA and RNA from the palms and fingers of male and female volunteers. Through mRNA profiling they demonstrated that about 15% of samples included non-skin cells. Strikingly, the prevalence of mixtures with a foreign DNA contribution of more than 20% (based on peak height comparison) was 19.2% but when considering mixtures with at least one foreign allele that percentage grew to 46.7%. van den Berge, Ozcanhan, Zijlstra, Lindenbergh and Sijen (2016) found an even higher incidence of foreign alleles of 88%, with samples yielding 0-67.5ng of DNA and ranging from 1-4 contributors. Similarly under fingernails they found 0.3ng-58.6ng of DNA with 80% of samples showing background alleles (1-3 contributors).
While the origin of the foreign DNA in both studies was not known, both suggest that the hands are indeed a vector for indirect transfer and that a foreign DNA contribution to a mixed profile of up to 20% relative ratio may not be unexpected. In these studies, relative peak heights seem to provide a basis to asses the likelihood of alleles belonging to the background or to the donor profile with very low quality samples however, peak heights may fluctuate and complicate this approach. Of particular interest is the observation that only 32.1% of mixtures presented with all alleles of the donor (Lacerenza et al., 2016) as this could blur the line between foreign and donor alleles when dealing with trace DNA samples.
3.1.2
Foreign trace DNA on clothing
As clothes are involved in most human interactions they are of high interest to criminal investigation. Although less relevant as a vector for transfer, scenarios involving clothing as a recipient of DNA are common. For example, when evaluating DNA recovered from a garment after an assault, the level of foreign trace DNA usually present on clothes becomes vital in assessing the value of the DNA evidence.
The prevalence of foreign trace DNA - A review of the available data
van den Berge et al. (2016) examined winter gloves in their wide ranging survey of private and public items. Gloves were taken from the users dominant hand and sampled on the finger and thumb area. In addition to this, van den Berge et al. (2016) took samples from trousers’ right and left ankles and from the armpits of shirts. Gloves produced 0.00-2.5ng of DNA from 1-4 contributors, with 90% of samples showing background alleles. Trouser ankles showed 0.1-29.0ng DNA with 2-5 contributors (88% with background alleles). These results align with the literat-ure on the prevalence of foreign DNA on hands and suggest that, like hands, gloves also contain at least a low level of foreign DNA from multiple donors. In contrast to gloves, shirt armpits only produced 0.00-0.06ng DNA, however surprisingly showed 1-3 contributors where 100% of samples showed background alleles. van den Berge et al. (2016) further note that in most cases the owner of the clothing was deducible as the major contributor and that mRNA typing mostly resulted in skin cell char-acterisation.
A similar pattern of multiple DNA contributors on clothing was observed by No¨el et al. (2016). They examined DNA on children’s underwear that was donated by different families after normal use and laundering. Swabs from the underwear did not produce DNA profiles, but cuttings generally showed mixed profiles from mul-tiple members of the family. No¨el et al. (2016) attributed this mostly to transfer through laundering, however due to the experimental setup they would also have observed any background DNA in general.
No¨el et al. (2016) offers an indication for the possible origin of foreign DNA on gloves and trousers (van den Berge et al., 2016), namely members of the immediate social group. But where in van den Berge et al. (2016) and Lacerenza et al. (2016) foreign DNA contribution was clearly distinguishable from the major donor by peak heights, relative contributions by family members seem to vary wildly (No¨el et al., 2016). This presents a clear problem when assessing cases of sexual assault within a family but also suggests a difference in the type of foreign DNA on the garments. It could be argued that underwear, unlike gloves and trousers, are rarely exposed to the environment and thus less likely to pick up DNA from outside the immedi-ate social group. Conversely, outer garments should have a higher contribution of environmental DNA.
How much DNA clothing picks up from the environment was studied by Ruan, Barash, Gunn and Bruce (2017). Shirts were freshly laundered before daily activ-ity, and 22-38% produced DNA mixtures with 14 or more alleles from additional contributors that were uploadable to a DNA database. After an average wear of approximately 9h, the shirts (n=150) showed an 8 fold increase in total DNA, with 20-26% uploadable contributor profiles. While the percentage of uploadable profiles decreased, and thus the increase in total DNA could be explained as self DNA, Ruan et al. (2017)’s other observation show that foreign allele number and peak heights also increased. Generally the shoulder and chest areas increased in wearer DNA, while mixed profiles increased on the back area. This strengthens the idea that outer garments pick up DNA from the environment and thus likely from a much larger group than just the family.
The prevalence of foreign trace DNA - A review of the available data
Figure 3.1: Average number of contributors to DNA profiles on public objects. Shown with one standard deviation. Data from van den Berge, Ozcanhan, Zijlstra,
Lindenbergh and Sijen (2016).
Additionally, Ruan et al. (2017) demonstrated Y-chromosomal alleles in 37/69 before wear and 49/69 after wear samples of female participants. As the authors note, this presents problems in the interpretation of Y-chromosomal markers which are often used in sexual assault cases. As with other foreign DNA though, exper-iments determining the peak height ratio between Y-chromosomal DNA deposited during assault and that observed from the background may hold a solution to this problem.
3.1.3
Foreign trace DNA on public objects
Clothes are not the only item that are encountered often in forensic casework. A host of items may be relevant in a case, for example because a DNA sample on them is alleged to predate the crime related activity or is hypothesised to be involved in indirect transfer. As Champod (2013) points out, the scientist evaluating such hypotheses should use as much relevant data as possible.
van den Berge et al. (2016) addressed this need and surveyed public objects that people frequently interact with. The list of objects includes escalator/stair rails, door handles, toilet buttons, shopping cart handles, library books and money. While they report more detailed DNA and mRNA typing results for each individual item, in summary they found 0-41.1ng DNA with 0-6 donors on these objects. In 17% of cases a major donor was deducible and mRNA typing showed between 0-3 cell types, with skin as the most frequent result.
There are two considerations that follow from these observations. First, everyday objects may serve as readily available vectors for foreign DNA pick-up by for example the hands. As can be seen from figure 3.1 public objects hold DNA from many contributors. This could partially explain the variety of DNA found on hands by Lacerenza et al. (2016) and van den Berge et al. (2016), as most of the object
The prevalence of foreign trace DNA - A review of the available data
surveyed are frequently touched during daily life. Second, objects not related to a crime may appear otherwise. A door handle, for example exhibiting a mixed DNA profile with a clear major contributor (van den Berge et al., 2016), may be mistakenly interpreted as a door handle with background DNA and a crime-related, or recent, major contributor. It may be concluded that a major donor (van den Berge et al., 2016) was the last handler of the object but it may also be true that this is a frequent user instead of the most recent one. Studies on the relative ratio of frequent handlers to recent ones are clearly needed. There is however a general question of relevance with such public, highly frequented objects and van den Berge et al. (2016)’s findings validate the general caution applied to them.
3.1.4
Foreign trace DNA in police laboratories
Even highly controlled areas such as police laboratories seem to be affected by the prevalence of foreign DNA. Fonneløp, Johannessen, Egeland and Gill (2016) demon-strated foreign trace DNA on various items and surfaces in a police lab and detected 16 previously unreported contaminations through officers. Some working on the case in question and some being unrelated to said case. Similarly, Taylor, Abarno, Rowe and Rask–Nielsen (2016) performed extensive environmental monitoring in a forensic laboratory and found that people do shed their DNA, in variable amounts, in areas they frequent but also in areas they do not frequent. They did demonstrate that office items, such as chairs, often hold DNA of 5 or more people. But more interest-ingly, they also recovered DNA of up to 3 people from laboratory equipment, and up to 5 people from case files.
Despite contamination prevention protocols, trace DNA may thus, on rare oc-casions, be introduced to samples accidentally and could appear as foreign trace DNA. Likely however, there will be differences between labs and more studies are needed. Crucially, the applied contamination prevention protocols should be pub-lished alongside the data so specific causes of contamination may be deduced.
3.2
The origin of foreign trace DNA
In some cases specific scenarios are brought forward as hypotheses that explain the presence of trace DNA. Simply to refer to the general prevalence of DNA on an item in question may be too unspecific for an activity level analysis in such a case. Consequently, data on specific origins of foreign trace DNA is very valuable.
3.2.1
Foreign trace DNA through laundering
Instinctively, laundering in a shared washing machine may explain the presence of trace DNA on clothing. Unsurprisingly it is often considered as a hypothesis where shared washing machines are relevant to a case. Two mechanisms could cause such a DNA transfer: DNA build up in the washing machine or direct DNA transfer from garment to garment.
The prevalence of foreign trace DNA - A review of the available data
The former, DNA build up in washing machines, was tested in two studies. Voskoboinik, Amiel, Reshef, Gafny and Barash (2017) did not recover any DNA above a detection limit of 0.001 - 0.003ng/µl from public and private washing- and drying-machines by swabbing. No¨el et al. (2016), however, obtained different res-ults. In their study, 6 ejaculates were placed onto pristine bed sheets and washed. Swabs taken from the washer’s drum after laundering showed 0.4 - 2.4ng of DNA in the sperm fraction with 3 to 18 alleles. While it may be argued that washing of bed sheets with 6 ejaculates is not common, and sampling directly after wash-ing is not realistic, No¨el et al. (2016) at the very least proved the concept of DNA residue in washing machines. This unfortunately leaves open whether the amount of DNA, the time since the last wash or both are the cause of the discrepancy between Voskoboinik et al. (2017) and No¨el et al. (2016).
The second way of DNA transfer from one garment to the other via the washing machine was also investigated by No¨el et al. (2016). They could demonstrate that pristine underwear, co-washed with bed sheets holding varying numbers of ejacu-lates, will contain up to 1.2ng of male DNA and can amplify 0-28 alleles. Notably, 7 randomly selected garments produced negative results for PSA and AP test, in-dicating that they would fall under the trace DNA category in casework. They observed similar results for pristine underwear washed with worn underwear from female donors. While swabs did not meet the detection limit, cuttings from the pristine underwear showed 0.2 - 3.2ng of DNA.
In addition to DNA from the sperm fraction, No¨el et al. (2016) sometimes observed the female partner’s DNA profile in the epithelial fraction. In contrast, Kamphausen, Fandel, Gutmann, Bajanowski and Poetsch (2015) concluded that transfer of epi-thelial abrasions from one garment to another during washing does not occur with noteworthy quality and quantity. However, Kamphausen et al. (2015) did recover 210pg - 2ng (hand-wash) and 140pg - 870pg (machine wash) DNA from garments which may very well be relevant under trace DNA conditions. Similarly van den Berge et al. (2016) observed mRNA results for skin and sometimes few alleles after washing blank fabric pieces with normal laundry. The results of No¨el et al. (2016) and Kamphausen et al. (2015) thus do not necessarily contradict each other, es-pecially when one considers that the epithelial fraction (No¨el et al., 2016) will for example include vaginal mucosa. Further corroboration is provided by Voskoboinik et al. (2017). Here, pristine socks washed together with a family’s general laundry showed DNA in 22% of cases (detection threshold 0.006ng/µl). Moreover, 6 out of the 7 samples with detectable DNA produced single source or mixture profiles.
There is some indication however that not all body fluids lead to DNA transfer during laundry. van den Berge et al. (2016) measured DNA yields from textiles spotted with saliva or blood before and after washing, the persistence rate was only 0.001% . Nonetheless, incomplete profiles with a maximum of 9 alleles were produced.
It is likely that factors such as the washing cycle or detergents influence results, however the literature indicates that laundering can lead to incomplete to full mixed or single source DNA profiles in a considerable number of cases. The discrepancies
The prevalence of foreign trace DNA - A review of the available data
between studies however suggest a high variance, which may be particularly related to sexual activity of household members. Moreover, laundering with other garments, rather than DNA buildup in the washing machines seems responsible for said DNA transfer.
3.2.2
Foreign trace DNA through evidence handling
As indicated previously (section 3.1.4), even police laboratories have a presence of trace DNA. Investigative efforts themselves may thus accidentally become a source of foreign trace DNA.
Following a suspicion that evidence bags received at the front desk of the police laboratory may be a vector of foreign trace DNA, Fonneløp et al. (2016) deposited clean swabs and fabric pieces inside of evidence bags. The bags were then handled without gloves by a known individual and opened by an analyst wearing full pro-tective equipment. One out of 60 swabs showed a full DNA profile of the known, unprotected handler, as did 1/20 of fabric samples. 11 fabrics however showed par-tial profiles with 0-6 alleles over 200RFU (10-29 above 30RFU).
Fonneløp et al. (2016)’s findings strongly suggest that even if lab personnel wear extensive protective equipment, foreign trace DNA may be deposited on exhibits via the packaging. Even accounting for elimination databases extending to front-desk personnel, which would likely prevent false attributions, the evidential value of the DNA could be compromised as loci consistent with the profile of the contaminating person would have to be disregarded. On top of that, transfer could also happen from other items to the packaging and then to the exhibit. A case file for example might be stored, or transported, together with an evidence bag. This is especially concerning considering the earlier discussed study by Taylor et al. (2016) in which a host of items in a police laboratory exhibited considerable levels of foreign trace DNA. That DNA could extend to old, or parallel, cases thus creating a real chance of falsely implicating someone in a crime.
3.3
The composition of trace DNA left behind
after contact
The prevalence of foreign trace DNA and its potential sources are important to infer the background level of trace DNA. In the case of foreign trace DNA on hands one might even be able to infer, to some degree, the composition of a trace DNA left by touch. The mechanism(s) involved in DNA transfer however may not result in an exact representation of the DNA of the transfer vector. To understand how much and which DNA can be expected after a specific contact, data from transfer experiments is indispensable.
3.3.1
Transfer through touch
Manoli et al. (2015) sampled plastic tubes held by volunteers for 5m after their normal daily activity. Profiles showed foreign alleles in 49% of samples (17% with 4
The prevalence of foreign trace DNA - A review of the available data
Figure 3.2: Percentage of samples with at least one foreign allele on hands and deposited by touch.
Data as reported by the indicated studies. Note that experimental setup and data reporting may differ between studies.
or more alleles) and foreign alleles could usually be distinguished due to their lower peak heights. This suggest that touching of (pristine) objects may generate traces that are alike to those recovered directly from hands. More corroboration of this pattern comes from a notable proxy of trace DNA deposited via touch, fingerprints. Templeton, Taylor, Handt and Linacre (2017) recovered DNA from fingerprints after powder enhancement and 49 out of 160 samples produced mixed profiles despite using fresh powder enhancement materials between experiments. Similarly, Lim, Subhani, Daniel and Frascione (2016), who recovered DNA from fingerprints on metal cables, observed foreign alleles in 31.8% of samples (1-7 alleles). M. Goray, Fowler, Szkuta and van Oorschot (2016) who investigated DNA from hand prints deposited onto glass made another interesting observation. Foreign DNA (more than 1 allele not from donor) was detected in 74% of samples with an average contribution of 26%, again generally observed as the minor component in a mixture. Surprisingly though, in 2.9% of samples the hand print donor themselves were excluded from the mixture.
Unsurprisingly, these observations are in line with the findings for trace DNA on hands, see figure 3.2 Some level of foreign DNA contribution may be expected from touch with hands, however in many cases it can be distinguished by peak heights. A roughly 3% self-exclusion chance of the donor themselves though could lead to misleading conclusions that could falsely implicate someone as the person who de-posited the DNA sample in question. This would in essence be similar to incidental inclusions, however with the added confusion that the real donor themselves would not be identifiable from the sample even if they were compared to it.
3.3.2
Transfer through touch after handshake
Certain actions prior to transfer may increase the level of foreign trace DNA origin-ating from a specific person. This becomes all the more important when such an
The prevalence of foreign trace DNA - A review of the available data
explanation is used in a criminal case. A defendant may, for example, claim that the presence of his DNA is entirely the result of shaking hands with the real donor of the trace. The scientist charged with the interpretation should then use data specifically related to this scenario (Champod, 2013). Some studies that focus on handshakes in particular are now available.
Cale, Earll, Latham and Bush (2015) paired participants and let them shake hands for 2m after which they handled knives which were extensively decontamin-ated. 85% of samples from knife handles showed the second handshake participant, and even though participants rinsed their hands and wore gloves prior to hand-shaking, some samples showed completely foreign alleles. This shows not only the possibility of foreign DNA transfer originating from a handshake but also highlights the persistence of foreign DNA on hands despite hand washing. Szkuta, Ballantyne and van Oorschot (2017)’s study on hand-prints left on 5 subsequent glass plates after handshakes however show the variance in trace composition. On the first plate the handshake partner was not excluded from the DNA mixture in 6/12 cases and in 5/12 cases the trace donor was the major, or even only, contributor. In line with M. Goray et al. (2016) experiments, one mixed profile even lead to the exclusion of the donor. On subsequently touched plates 16% of cases excluded the donor and 80 and 87% respectively excluded the handshake partner. If 15m elapsed between the handshake and hand-print on the first plate, the donor was the major, or only contributor, while the known contributor was not excluded from 3 plates.
Szkuta, Ballantyne, Kokshoorn and van Oorschot (2018) on the other hand directly mimicked a case scenario. They had participants shake hands and then perform nor-mal activities for 40m-8h until firmly handling an axe. The depositor was the major component in all cases except one and the handshake partner was demonstrable up to 5h after the handshake.
Instead of using pristine objects G. E. Meakin, Butcher, Oorschot and Morgan (2017) let volunteers handle knives for two days, then had them handshake another participant and stab a foam block for one minute. Prior to stabbing the regular user was the major component in the profile, rarely with foreign alleles (1-3% of the profiles). After stabbing, profiles from knives showed alleles from the handshake partner in 3/4 of cases in an approximately 1:10 ratio with the regular user of the knife.
Even under controlled conditions the trace composition after handshakes varies widely, but the handshake partner’s DNA seems to be detected frequently. In many cases though, the donor themselves continue to be distinguishable by peak heights. The more complex the situation gets however, e.g. due to elapsed time, the more complex the traces seem to become. Though, foreign trace DNA from handshakes looks to be deposited long after the handshake took place. Not considering peak heights, this persistence would have a considerable influence on strength of evidence in favour of the handshake scenario.
The prevalence of foreign trace DNA - A review of the available data
3.3.3
Transfer through touch after non-intimate social
con-tact
Similar to handshakes, non-intimate social contact such as hugging, light massages etc. could be relevant in casework and data on trace DNA composition after these action are thus needed. Zoppis et al. (2014) tested the composition of trace DNA left by touch after touching sebaceous and non-sebaceous skin areas of a person. Before hand-washing they report ’typical contamination phenomena’ in trace DNA recovered from a touched glass plate. After washing hands, hand-prints following touch of non-sebaceous skin showed no profiles. Hand-prints after touching se-baceous skin however resulted in 5 full, single profiles, one non-interpretable and one partial profile and some background contamination. Although the sample size in this experiment is limited, it suggests a difference in the persistence of foreign trace DNA between touch of sebaceous and non-sebaceous skin. Jones et al. (2016) simulated non-intimate social contact by letting a male and female participant rub their hands together and/or the male massaging the shoulders and neck of the fe-male. The male then simulated urination immediately or 6h after contact with the female. After 6h, no notable amount of female alleles were observed. Immediately after simulating urination 5 of 30 waistbands of underwear showed female alleles. Additionally, 4/30 samples taken from the penis did so as well. The waistband showed a maximum of 11 alleles with 180-860RFU and the penis 1-5 alleles with 85-160RFU, none of the samples from the inside of the underwear showed females alleles.
Both of these studies show that social, or more generally, skin contact can lead to similar trace DNA phenomena as handshakes. Zoppis et al. (2014) however suggest that the area of skin touched may influence the composition of the trace. As with other transfer mechanisms and objects, Jones et al. (2016) shows low peak heights, suggesting that foreign trace DNA transferred by touch may be distinguishable from the donor by peak heights. In their specific application to sexual assault cases the same is likely true for traces from social contact compared to those generated through sexual intercourse.
3.3.4
Transfer by touch after touching previously used
ob-jects
Considering the prevalence of foreign trace DNA on many objects, contact traces left behind after handling such objects may be hypothesised to contain more foreign trace DNA than normal. The only study on the topic however focuses on the possibility of trace DNA transfer during evidence handling and scene investigation. Fonneløp, Egeland and Gill (2015) modelled transfer of DNA at the crime scene via an investigator’s gloves. Wooden, plastic and metal objects were handled by known donors for 30s after washing their hands. Subsequently, a gloved investigator handled them for the same time, after which he handled a piece of fabric. Additionally, the experiment was repeated for 10s handling times. Not only did Fonneløp et al. (2015) demonstrate full profiles of the known donor on all 3 substrates with only 10s handling time, but in addition 17/108 samples showed unknown alleles.
The prevalence of foreign trace DNA - A review of the available data
In conjunction with the prevalence of trace DNA on, for example, door handles (van den Berge et al., 2016), but also on forensic-staff’s equipment (Fonneløp et al., 2016), this is very concerning for forensic investigation. Without extensive de-contamination protocols and frequent change of protective equipment investigators may inadvertently transfer trace DNA to otherwise pristine items which may then be falsely interpreted. More generally though this study shows that, indeed, pick up of trace DNA may be very relevant to trace DNA interpretation and there are some arguments to consider contamination in mathematical models.
3.3.5
Transfer through touch after contact with used
tex-tiles
Helmus, Bajanowski and Poetsch (2015) had volunteers deposit their DNA onto textiles by rubbing them on their neck. The cloth was then rubbed against a second persons hand who in turn rubbed cotton cloths and plastic bags. Full, or partial, profiles of the first person were recovered from both the plastic bags and cotton cloths (25% and 62% respectively).
Unsurprisingly these results suggest that, like other objects, textiles are a vector for trace DNA pickup. The available data are however severely limited and should be used only with the appropriate caution.
3.3.6
One time use of regularly used objects
Unlike in other experiments, in real casework regularly used, rather than pristine, object will often be encountered. It is thus important to understand the DNA composition of traces recovered from such objects in particular.
Oldoni, Castella and Hall (2016) tested the relative contribution of a one-time user to the DNA profiles from objects that had a known, regular user. This included a variety of plastic, metal, nitrile and fabric items. Considerable variance in the ratio of regular user to one time user DNA was observed. However, the one time user was the major component in mixed profiles in 15% to 55% of cases, depending on the type of item. 71% of profiles also included additional foreign alleles as minor components. Breathnach, Williams, Mckenna and Moore (2016) also studied the ratio of habitual user to one time user but specifically for underpants. Underpants were worn by males for 12h and then touched by a female volunteer for 15s. 61.9% of waistband samples yielded interpretable DNA profiles with 87.0% of those showing the wearer as a single profile or as a major contributor in a mixture (except in one case). The female participant was detected in only 11.1% of samples and was present as the major, or only contributor, in 9.53% of cases. Additional foreign background DNA were observed in 87.3% of all cases but were only reportable as donors in 14.4%.
Both studies indicate that the ratio between one time and regular user can fluc-tuate considerably. In some cases the one time user is the only donor detected, in others the regular user is the only donor. In most cases however one, or both, are part of a mixture, often with foreign trace DNA as a minor component. Imagine a
The prevalence of foreign trace DNA - A review of the available data
scenario where a handshake takes place, after which somebody uses a knife that is normally used by a third person. There is a probability that the handshake part-ner is detected as a minor component, but also an, albeit small, chance that he is the only contributor. Now add to that the complicated relationship between this toucher-DNA and the regular user DNA. This illustrates how incredibly complex trace DNA interpretation can become and why complex statistical tools such as Bayesian networks (Biedermann & Taroni, 2012) may be required.
3.3.7
Complex scenarios
Cases in which trace DNA interpretation is relevant can be extremely complex. They may involve a number of variables at the same time, such as multiple handlers, regular users etc. Some studies model these highly specific scenarios and serve as an illustration of the complexity involved.
Buckingham, Harvey and Oorschot (2016) examined a very complex scenario of 4 volunteers handling the same knife in sequential order. While they did observe a general increase in DNA of later handlers of the knife, the last handler was not always the major contributor. Additionally, they observed varying relative contri-bution between participants but also between experiments with the same conditions (including the same participant). Simulating an actual attack Fonneløp, Ramse, Egeland and Gill (2017) sampled worn T-Shirts after a known contributor pulled the wearer back on their shoulders while they tried to run forward. They found that the, previously determined, high-shedder status of the person was significantly cor-related with the frequency of detecting the attackers DNA. When the attacker was a high-shedder they were usually observed as the major contributor and conversely when they were low-shedders a major contributor often was not determinable. High-shedder victims on the other hand were frequently the major component whereas low-shedder victims were often minors or had no contribution attributed to them.
Steensma et al. (2017) let 20 volunteers tie 5 cable ties each and collected inform-ation on prior activities, living conditions and others. One cable tie per volunteer was then send out to different laboratories (total of 4 laboratories) to study how much the results would differ. Between the laboratories the proportion of mixtures differs (see figure 3.3). Steensma et al. (2017) did not find trends between foreign trace DNA presence and recent physical content. They do note though that as time since the last hand was decreases, the proportion of observed mixtures increases (data set did not allow for statistical tests). They further note that differences in laboratory protocols may lead to differences in interpretation standards which may explain some of the discrepancies.
While on the one hand this shows that increasingly complex scenarios become increasingly hard to interpret, these studies also highlight that individual difference, and even differences between laboratories, may play an important role in the com-position of trace DNA. To get an idea of the complexity of transfer under real life conditions the interested reader is referred to Mariya Goray and Oorschot (2015) who studied DNA transfer during unscripted social interaction. Not only does this study indicate the enormous number of steps that may be involved in DNA pickup
The prevalence of foreign trace DNA - A review of the available data
Figure 3.3: Composition of DNA profiles recovered from cable ties by 4 different laboratories
Data reported by Steensma et al. (2017).
and transfer but it also reaffirms that in most cases the last person to come in contact with an object is often the major contributor. However, as in Buckingham et al. (2016) this is not always the case and even contact that would intuitively to lead to transfer sometimes shows no DNA at all. In reality peak height difference may thus serve as a guide, especially in less complex scenarios, but should likely be approached with caution.
3.4
Summarising discussion
As established by Champod (2013) and Casey et al. (2016), activity level interpreta-tion of DNA evidence should be based in relevant data were possible. G. Meakin and Jamieson (2013) back in 2013 concluded that the available data was not sufficient for such an interpretation, since 2013 however, a number of papers on trace DNA have been published. For most activity level interpretations one or more hypotheses will consider the chance of indirect transfer or background levels of foreign trace DNA. Data on this topic is thus extremely relevant.
In total, 29 different studies that consider foreign trace DNA were identified in this review. All of them publish numerical data that could in principal be used in an activity level analysis. Some even publish complex analysis frameworks in the form of Bayesian networks alongside their data (see for example Szkuta et al. (2018)). This shows the continued interest and need for activity level analysis of trace DNA by the forensic community and stakeholders alike. It is the author’s opinion however, that several problems remain despite the growth in available data.
The prevalence of foreign trace DNA - A review of the available data Table 3.1: Numbers of foreign trace DNA studies published since 2013
Broad topic Specific topic Number of studies (Studies may cover multiple topics) The prevalence of foreign trace
DNA
Foreign trace DNA on hands 2 Foreign trace DNA on clothing 3 Foreign trace DNA on public objects 1 Foreign trace DNA in police laborator-ies
2
Origin of foreign trace DNA Foreign trace DNA through laundering 3 Foreign trace DNA through evidence handling
1
The composition of trace DNA left behind by contact
Transfer through touch 4 Transfer through touch after a hand-shake
4 Transfer through touch after non-intimate social contact
2 Transfer by touch after touching previ-ously used objects
1 Transfer through touch after contact with used textiles
1 One time use of regularly used objects 2 Complex scenarios 4
As can be seen from table 3.1, the 29 papers are split between various topics. Of course that is not surprising, however leads to the problem that data in any given category are rarely reproduced. This lowers the reliability of the data considerably. In addition, many of the study’s sample sizes are quite small, which will likely amplify this effect. On top of that, no reporting standards for the presence of foreign trace DNA exist. Across the literature data are reported in a multitude of ways. This makes it harder to combine data and further lowers their potential. Where there are multiple studies on similar topics however, for example on foreign trace DNA by touch and on hands, trends in the data are visible (table 3.2).
Even when considering these trends one has to take into the manner in which the data were measured. For data to be widely applicable across laboratories and cir-cumstances the largest variance possible should first be captured. Then the influence of specific factors can be studied to narrow down the variance when circumstances are known. Instead, studies on foreign trace DNA often seem to go the opposite dir-ection. Very specific circumstances are studied and applied to very specific, mostly fictional case scenarios. For example, data seem to be generally derived from a small group of volunteers in a laboratory environment, rather than from daily life. Foreign DNA pickup by volunteers in a DNA laboratory is likely much lower than that in, for example, a shopping mall. If the scientist would now apply foreign trace DNA transfer probabilities from the laboratory scenario to the shopping mall, the value of the DNA evidence under the defence hypothesis would likely be underestimated. If data captured the largest variance instead, an activity analysis may overestimate the value of evidence in favour of the defence hypothesis, however this would be in line with the in dubio pro reo principle. This would require large sample sizes with study subjects in a large variety of situations.
The prevalence of foreign trace DNA - A review of the available data Table 3.2: Comparison of foreign DNA prevalence after contact of interest.
Study Transfer mechanism Foreign alleles Foreign as minor Manoli et al. (2015)
By touch
49.00% Yes Lim, Subhani, Daniel
and Frascione (2016)
31.80% -Templeton, Taylor,
Handt and Linacre (2017)
30.60%
-M. Goray, Fowler, Szkuta and van Oorschot (2016)
74.00% Yes
Cale, Earll, Latham and
Bush (2015) By touch after handshake
85% Sometimes G. E. Meakin, Butcher,
Oorschot and Morgan (2017)
75% Yes
Fonneløp, Egeland and Gill (2015)
From touched object
to other object via gloved hand
93.30% (known donor), 15.70% (unknown)
N/A
Helmus, Bajanowski and Poetsch (2015)
By touch after
rubbing used textile 25% - 62% (substrate de-pendent)
N/A
Understandably though, studies of this extend are unrealistic for small casework laboratories. This could only be overcome by large scale cooperation between labor-atories and should likely involve bodies such as ENFSI. Ideally, a large, shared database would be set up, in which studies, and crucially data reporting, are stand-ardised. From this database probability assessments for evidence under given hy-potheses could be derived. Over time the influence of specific factors on these probabilities could be studied and incorporated into assessments.
In the interim, scientists will continue to be called upon to interpret trace DNA at activity level. For their evaluations they can now refer to a growing body of knowledge. Champod (2013)’s warning to stress the limitations of the interpretation, however, is now more relevant than ever. Since 2013 data on foreign trace DNA has become available, though it remains highly situational and shows large variance across studies. In the authors opinion, extreme care should be taken should any of the data be applied in a court of law. Ideally, the data would only be used as a general indication of the strength of evidence rather than in strict numerical assessments. The former would force the scientist to explain limitations, while the latter may suggest a false level of certainty. In addition, counter-intuitive scenarios, such as the donor a hand-print getting excluded from its DNA profile, should be discussed where appropriate.
Chapter 4
Conclusion
Since G. Meakin and Jamieson (2013) conducted a number of papers on foreign trace DNA have been published. From the literature the following conclusions can be made:
The prevalence of foreign trace DNA Items such as hands, clothing, door handles, money, books and others generally carry foreign trace DNA. The number of donors can often reach three people and the data indicate that foreign trace DNA is, in most cases, distinguishable from regular users of the object by peak height differences.
The origin of foreign trace DNA Laundering with other garments is likely one of the factors contributing to foreign trace DNA on clothing. During police investig-ations and laboratory analysis foreign trace DNA may be introduced inadvertently. Other origins of foreign trace DNA remain unstudied. However, foreign trace DNA is a result of complex human-human, human-object and object-object interactions that may be extremely difficult to study.
The composition of trace DNA left behind after contact Generally traces left behind after a contact seem to be reflective of data on foreign trace DNA on items. Foreign DNA is often transferred (up to 30-90% of samples show foreign alleles) but full profiles rarely occur. Commonly they can be distinguished from the donor by peak height comparison and less frequently donors cannot be distinguished or may, in even rarer cases, not show as contributors to their own traces.
Recommendations
• Current data should only be used as a general indication of the strength of evidence. Limitations should be addressed in reporting and in the court room. • Counter-intuitive scenarios such as exclusion of the donor of a trace from a
mixture should be considered.
• Studies on foreign trace DNA should involve larger sample sizes and try to capture as much real life variance as possible.
• Experimental setups and data reporting should be standardised where pos-sible.
• Laboratories should cooperate in order to form large scale databases for data on trace DNA.
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